Potentiometric titration of strong acid against a strong base
To Determine Normality of given acid by potentiometry
In potentiometric titration, absolute potentials or potentials with respect to a standard half cell are not usually required and measurements are made while the titration is in progress. The equivalence point of the reaction will be revealed by a sudden change in potential in the plot of emf readings against the volume of the titrating solution, any method which will detect this abrupt change of potential may be used. One electrode must maintain a constant, but not necessarily known, potential; the other electrode must serve as an indicator of the changes in ion concentration and must respond rapidly. The solution must of course be stirred during the titration. A simple arrangement of potentiometric titration is given in the figure below. A is the reference electrode, B I the indicator electrode and C is the mechanical stirrer; the solutions to be titrated is contained in the beaker.
For Strong acid contents determination.
- Prepare solutions of acid and base, each approximately 0.1 M, and set up a pH meter as described above.
- Fit up the apparatus shown in figure above with electrode assembly (or combination electrode) supplied with the pH meter supported inside the beaker. The beaker has a capacity of about 250 or 500 ml and contains 50 ml of the solution to be titrated (acid sample)
- Select a burette and fill it with prepared sodium hydroxide solution taking care to remove all air bubbles from the capillary extension, and then clamp the burette so that the end of the capillary is immersed in the solutions to be titrated. This procedure ensures that all additions recorded in the burette have in fact been added to the solution, and no drops have been left adhering to the tip of the burette, a factor which can be of some significance for emf readings made near the end point of the titration.
- Stir the solution in the beaker gently. Read the potential difference between the electrode with aid of the meter. Record the reading and also the volume of alkali in the burette.
- Add 2-3 ml of the solution from the burette, solutions from burette; stir for about 30 seconds and after waiting for a further half-minute measure the emf of the cell.
- Repeat the addition of 1 ml portions of the base, stirring and measuring the emf after each addition until a point is reached within about 1 ml of the expected end point. Henceforth, add the solution in portions of 0.1 ml or less and record the potentiometer reading after reach addition. Continue the additions until the equivalence point has been passed by 0.5 -1.0 ml.
- Plot the potential as ordinates and volumes of reagent added as abscissa. The equivalence point is volume corresponding the other steepest portion of the curve. In some cases the curve is practically vertical, one drop of solution causing achange of 100-200 mV in the emf of the cell; in other cases the slope is more gradual.
Indian Pharmacopoeia, Govt. of India, 2007, Volume 1, P- 142
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